When Atonement was first published, reviewers, not wanting to spoil a surprise, failed to mention its metanarrative. This is the narrative that explains its narrative - the story about how its story comes into being. On page 349 of the British edition, we reach the end of Briony's wartime visit to her sister, Cecilia, and her lover, Robbie, in Balham, south London.

Briony parts from the couple at the tube station, having promised that she will write to her parents and to lawyers admitting that she lied some six years earlier when she testified to Robbie being a rapist. "She knew what was required of her. Not simply a letter, but a new draft, an atonement, and she was ready to begin."

What is this "new draft", this "atonement"? At the foot of the page is printed:

BT

London 1999

The first-time reader is likely to be puzzled. But then the place and date are repeated as the heading for the next section of the novel, "London 1999". Here Briony is the first-person narrator, on her 77th birthday. You realise that those initials at the foot of the previous section ("BT" is Briony Tallis) mean that the whole of the rest of the book was Briony's story - her work of fiction. She has been writing it and rewriting it for the previous 59 years. We find out that it has gone through "half a dozen drafts". The latest one is what we have just read. The novel is composed of this story and the metanarrative - the short final section - that accounts for its existence.

It is perhaps something like the discovery near the end of the film The Usual Suspects that all that has gone before has been the invention of one of the characters. Except that, even on first reading, we might notice the clues that McEwan has given to the provenance of the narrative. Young Briony is an aspiring novelist, always "writing stories". Her sharpest experiences are those that seem worthy of being turned into writing. When she imagines anything, it is to think "how she might describe it". Working as a trainee nurse, she finds secret moments to fill her notebook with fictionalised elaborations of her experiences on the ward.

McEwan's device is time-honoured. Cecilia Tallis is whiling away the hot summer after leaving university by reading Samuel Richardson's Clarissa. Robbie keeps asking her how she is getting on with it. Richardson's novel, too, tells the very story of its own existence: you find out at its conclusion that the "letters" that comprise the book are Clarissa's "legacy" after her death. The heroine commands in her will that they be published. Did McEwan have this in mind when he decided to make his own novel the novelistic testament of one of its characters?

Metanarrative should not be confused with metafiction, though the latter often contains versions of the former. "Metafiction" has become a label for novels about novel writing. Often it is attached to supposedly post-modernist fiction, though much earlier works seem to fit into the category. A notorious case is Laurence Sterne's Tristram Shandy. Metafiction is persistently self-referential, like Italo Calvino's If on a Winter's Night a Traveller, which, from its first sentence, makes a story out of how reader and writer conspire to make a story.

McEwan's metanarrative, however, is a revelation withheld. Some readers have felt cheated by it, like viewers of Dallas who were suddenly shown by desperate scriptwriters that the traumatic events of many previous episodes were just Pam Ewing's "dream". The test of the trick is perhaps in the rereading. It is impossible the second time around, even knowing what is to come, not to believe in the characters and situations as thoroughly as on the first reading.

Unlike writers of metafiction, McEwan wants you to identify with characters, to succumb to narrative illusion. For Briony to undertake her "atonement", her work of fiction must make up for, and confess, the wrong that she has done. In a novel, she can make the world better than it truly is. She can make Cecilia and Robbie survive and meet again. And we must be allowed to believe it.

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